Bonding a Ceramic Crown with a Moisture-Contaminated Field
A 52-year-old patient is receiving a lithium disilicate crown on a maxillary first molar. The assistant has prepared the tooth with a total-etch technique, applied etchant for 15 seconds, rinsed, and dried. The dentist is called away for 10 minutes, and during that time the assistant notices the prepared tooth surface has been contaminated by saliva from the patient's tongue and by air from the operatory's HVAC vent. The assistant has already mixed the resin cement and loaded it into the crown, which has been sitting on the tray for 6 minutes. The cement's working time is 2 minutes and setting time is 4 minutes. The assistant is unsure whether to re-etch, re-rinse, or proceed.
1. Assess the contamination and recommend the correct sequence of steps before cementation.
Read the worked answer
The tooth surface has been contaminated by saliva and airborne particles, which will compromise the bond. The assistant should not proceed with cementation. The correct sequence is to re-isolate the tooth, re-etch the enamel and dentin for the appropriate time (typically 15 seconds for enamel and 15 seconds for dentin with a total-etch system), rinse thoroughly, and dry gently without desiccating the dentin. A fresh coat of adhesive should be applied and light-cured according to the manufacturer's instructions. The pre-mixed cement that has been sitting for 6 minutes is beyond its working time and should be discarded because it has likely begun to set and will not flow properly. A new mix should be prepared only after the tooth is re-prepared and isolated. The HVAC vent should be addressed by redirecting airflow or using a rubber dam to prevent further contamination.
2. Explain how the assistant should manage the cement's working time and what factors affect it.
Read the worked answer
The cement's working time of 2 minutes means the assistant must have everything ready before mixing: the crown cleaned and tried in, the tooth isolated, and the light ready. Mixing should begin only when the dentist is ready to seat the crown. Working time is affected by ambient temperature, humidity, and the ratio of base to catalyst; a warmer operatory shortens working time. The assistant should follow the manufacturer's instructions for mixing and should not attempt to extend working time by adding more catalyst or by chilling the mix unless the manufacturer specifies. If the cement begins to string or lose gloss, it should be discarded. The assistant should also note that the crown should be seated within the working time and held with steady pressure until the initial set.
3. What documentation and patient communication should follow this event?
Read the worked answer
The chart should document the contamination event, the re-etch and re-isolation steps, the discarded cement, and the final cementation. The patient should be informed that the tooth surface was contaminated and that the assistant took steps to ensure a proper bond. The assistant should explain that the crown was re-cemented with fresh material and that the patient should avoid chewing on that side until the cement is fully set. Follow-up should include checking the crown's retention and margins at the next visit. The office should review the workflow to prevent similar delays, such as having a second assistant available or scheduling cementation when the dentist is not called away.